Ionizing true airspeed indicator



Sept. 6, 1955 A. A. STUART IONIZING .TRUE AIRSPEED INDICATOR Original Filed June is, 1948 2 Sheets-Sheet 1 wow INVENTOR. ALFRED 4. $7'UA/Q7 BY %Z(/ Array/w Sept. 6, 1955 A. A. STUART 2,716,891

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ALFRED fl. S 7' 0 41??" United States Patent 0 2,716,891 IONIZING TRUE AIRSPEED INDICATOR Alfred A. Stuart, Ridgewood, N. 3., assignor to Bendix Aviation Corporation, Teterboro, N. J., a corporation of Delaware Original application June 13, 1948, Serial No. 33,390, now Patent No. 2,679,162, dated May 25, 1954. Divided and this application October 1, 1953, Serial No. 383,465

6 Claims. (Cl. 73-194) This invention relates to a method and means for measuring the velocity of a flowing fluid medium, and more particularly to a true air speed meter for indicating the velocity of aircraft in the subsonic, sonic and supersonic ranges of airspeed. This application is a division of application Serial No. 33,390, filed June 18, 1948, issued as Patent No. 2,679,162 on May 25, 1954.

Prior devices for measuring the velocity of a fluid medium, excluding the vane and impeller types, relied upon the differential in the ambient or static pressure and the impact or dynamic pressure of the gas or liquid measured. Such measuring instruments were susceptible to serious errors due to the changes in the temperature and the density of the fluid medium. Various expedients were thus required to correct the readings of these meters.

In the airspeed meters used for indicating the velocity of an aircraft, compensation for air temperature and air density is required to obtain a true airspeed indication. The present airspeed instruments are normally provided with a differential pressure responsive member subjected to dynamic pressure on one side and static air pressure on the other. A pointer driven over a calibrated dial by the movement of the differential pressure member would show indicated airspeed in contradistinction to true airspeed. In order to compensate for the errors arising from varying ambient conditions, two aneroid capsules are ordinarily provided together with interconnecting linkages to correct the meter readings for the temperature and density variations of the air. Further compensation is also provided by the use of bimetal strips and/or pivots to correct for the ambient temperature of the instrument case.

While the present day airspeed meter, when compensated as indicated, does provide a fairly accurate reading of the true airspeed of a craft, the present development of the turbine and jet engines, and the use of rocket power to propel aircraft beyond the sonic barrier, have made the use of pressure responsive airspeed meters impractical. At subsonic, sonic, and supersonic speed levels attained by the new engines developed, and at the extreme high altitudes of flight now made and contemplated, the pressure responsive members as now used in airspeed meters are inaccurate and rupture easily.

It is, therefore, an object of my present invention to provide a meter for indicating the velocity of a fluid which is independent of the pressure of the flowing fluid.

Another object of my present invention is to provide a fluid velocity meter of the general character indicated which is independent of the temperature and/or the density of the flowing fluid.

Still another object of this invention is to provide a fluid velocity meter of the character indicated which shall be well adapted for use as a true airspeed meter for indicating the airspeed of aircraft at the subsonic, sonic, and supersonic speed levels and at all altitudes of flight level.

Yet another object of the invention is to provide a true airspeed meter for aircraft which shall be unaffected by fit) 2,7 l 6,8 9 l Patented Sept. 6, 1 955 shock waves encountered in entering and passing through the sonic speed barrier.

Still another object of my invention is to provide a true airspeed meter of the character indicated which shall not produce radio interference.

A further object of my present invention is to provide a fluid velocity meter of the character indicated in which the fluid medium is electrostatically charged, the velocity of the fluid medium being a measure of the time interval required for the charges to be carried between two known points.

Still a further object of my invention is to provide a fluid velocity meter of the character indicated in which the transit time of the electrostatic charges provides a time phase delay which is a measure of the velocity of the fluid medium.

Yet another object of this invention is to provide a fluid velocity meter of the character described in which the velocity of the fluid medium may be indicated directly in linear distance per unit of time.

Still another object of my present invention is to provide a true airspeed meter incorporating the features described which shall be automatic and positive in its operation, relatively inexpensive to manufacture, which shall consist of few and simple parts and circuit elements, light in weight and small in size, which shall have a large variety of applications, and yet be practical and eflicient to a high degree in use.

Other objects of this invention will in part be obvious, and in part be hereinafter pointed out.

In the illustrated embodiment I employ a novel electrode placed upstream in the fluid medium. The electrode is insulated from ground and supplied with a sufliciently high potential source to provide the electrostatic d charges. The high potential source is varied in a periodic manner and may be either positive or negative, or alternating. The receiving electrode is fixed downstream at a predetermined distance from the ionizing electrode. The ions collected by the receiving electrode will charge this electrode to a potential varying in the same manner as the potential impressed upon the ionizing electrode. The two potentials will vary in phase equal to the transit time of the ions. The induced potential of the electrode is amplified by an electronic circuit and applied to a phase measuring circuit whereby the true velocity of the fluid is indicated in linear distance per unit of time. In the description hereinafter given, this arrangement briefly described will be referred to as the phase shift method.

In carrying out my invention and applying the same to a true airspeed indicator for aircraft, I have found that serious radio interference is caused when the potential supplied to the ionizing electrode is of such a high value that a corona discharge takes place. The potential supplied to my novel ionizing electrode is maintained at the pre-corona level. As will hereinafter be more fully set forth, the ionizing potential required decreases with the decrease in atmospheric pressure. A pressure responsive member is included in my true airspeed meter to vary the ionizing potential to a pre-corona value as changes in altitude take place.

In the accompanying drawings forming a part of this specification, wherein similar reference characters designate corresponding parts throughout the several views,

Fig. l is a schematic wiring diagram illustrating an embodiment of the invention.

Fig. 2 is a longitudinal sectional view of a novel ionizing electrode, while Fig. 3 is a graph showing the ionizing and corona levels of potential with respect to atmospheric pressure. In the embodiment described and claimed herein, the

numeral designates a circuit interconnecting a pickup electrode 92 and a novel ionizing electrode 91, more fully shown in Fig. 2, and described and claimed in co-pending application Serial No. 132,125, filed December 9, 1949, now Patent No. 2,602,910, issued July 8, 1952. The ionizing electrode 91 comprises a. metallic, needle tipped electrode 93 incased in a sleeve 94 of insulating material, and a grounded metallic shroud 95 encircles the insulating sleeve. The sleeve extends a slight degree beyond the needle point of the electrode 93, the shroud 95 tapering oif toward the upper edge of the sleeve. The end of the ionizing electrode is placed in the slipstream of the fluid medium, the shroud and the sleeve insulating the electrode from its surrounding holder and preventing any arcing between the electrode and the adjacent surfaces.

The ionizing electrode 91 is connected by a lead 98 to the positive side of a grounded voltage divider comprising the two series connected resistors 99 and 100.

The voltage divider 99, 100 is connected across the secondary winding 101 of a high voltage rotary transformer 102, the purposes of which will hereinafter be described. The primary winding 103 of the transformer is connected across the output of a conventional power amplifier 104, here shown in block diagram form. The input of the power amplifier is provided by a conventional oscillator 105, the oscillator and amplifier being connected to a source of suitable power supply 106. A high, periodically varying voltage is thus supplied to the ionizing electrode 91 to permit ionization of the fluid medium. The potential supplied to the ionizing electrode must be varied in a periodic manner, and may be either positive or negative, or reversing.

It has been found in applying my invention to an airspeed indicator, that atmospheric pressure will affect the ionizing of the air. At pressures approximating atmospheric pressure at sea level, ionization of the air would require a potential of approximately 2200 volts R. M. S., no ionization taking place below this value.

An increase in the ionizing potential to approximately 4400 volts R. M. S. produces the required ions for the required ions for the satisfactory operation of the device as an airspeed indicator for aircraft. The increase of the potential supplied to the ionizing electrode beyond the latter figure causes corona discharge which produces severe radio interference.

As the ambient pressure decreases, the ionizing potential required decreases. The ionizing potential required at an altitude approximating an atmospheric pressure of 1 mm. Hg is about 300 volts R. M. 5., while 7' corona discharge will begin at a supply potential of about 400 volts R. M. S. at this pressure. Thus for increasing altitude of an aircraft, a zone of operation (Fig. 3) exists in which ions will be produced without corona discharge.

The rotary transformer 102 is provided to maintain the ionizing potential applied to the electrode 91 within the limits indicated in Fig. 3 to provide ionization without corona discharge. The secondary winding 101 of the transformer is capable of being angularly displaced with respect to the stationary primary winding 103. The secondary winding is coupled by suitable means indicated by the dashed line 108 to a pinion I09 and gear sector 110. The sector 110 is adapted to be oscillated by the expansion and contraction of an aneroid capsule 111 whose exterior surface is subjected to atmospheric pressure.

The changes in atmospheric pressure will, through the expansion and contraction of the aneroid, cause an angular displacement of the secondary winding 101 of the transformer with respect to the primary winding 103. w

The resultant decoupling of the two windings will reduce the voltage induced in the secondary winding. The potential supplied to the ionizing electrode 91 will be reduced as the altitude increases and will lie within the operational zone shown in Fig. 3 in which the required ions will be produced without the interfering corona discharge.

The pickup electrode 92 is placed downstream from the ionizing electrode 91. The electrode 92 is needlepointed and may extend to a slight degree into the slipstream. The electrode is suitably insulated from its support (not shown) and is electrically connected to the input of a volume compressor or limiter circuit 115 comprising the multi-element tubes 116 and 117 and described and claimed in co-pending application Serial No. 132,126, filed December 9, 1949, now abandoned.

The tube 116 is a variable mu tube preferably of the 6L7 type and comprises an indirectly heated cathode 118, a control grid 119, two screen grids 120 and 121 separated by a second control grid 122, and a plate 123. The tube 117 comprises an indirectly heated cathode 124, a plate 125, a control grid 126 and a second plate 127, the cathode 124 and plate 125 forming a diode section, the cathode 124, the grid 126 and the plate 127 forming a triode section.

The pickup electrode 92 is connected by a wire 130 to the control grid 119 of the tube 116. The control grid is connected through a lead 131 and a resistor 132 to the negative side of a battery 133; and by the lead 131 and a lead 134 to the negative side of a battery 135, the positive side of said battery being connected to the plate 125 of the tube 117. The cathode 118 is connected by a lead 136 to the negative side of a battery 137, the lead 136 being grounded as at 138. The screen grids 120 and 121 are internally connected and receive their bias supply from the battery 137 via a lead 139. The second control grid 122 is connected through a resistor 140 to the lead 131 and through a condenser 141 to the grounded lead 136. The plate 123 is connected by a lead 144 through the primary winding 145 of a transformer 146, to the positive side of the battery 137, a condenser 147 connected in parallel with the primary winding 145 forming a tuned circuit.

The control grid 126 of the tube 117 is connected to a secondary winding 148 of the transformer 146, the bias supply being a battery 149. A second secondary winding 142 of the transformer 146 is connected by leads 150 to a phase splitting circuit generally designated by the numeral 151. The plate 127 of the tube 117 is connected by a lead 152 to the positive side of the battery 133. The cathode 124 is connected via the lead 136 through a parallel connected resistor 154 and a condenser 155 to the negative side of the battery 133.

The tube 116 is operated as a remote cutotf pentode in which an extremely negative bias is required to cut ofi? all of the plate current. The normal bias for the control grids 119 and 122 is supplied by the battery 135 via the leads 134 and 131. The large bias for cut-off is supplied by the battery 133 in the manner to be described in connection with the operation of tube 117, the triode section of which is biased to cut-off by the battery 149.

The charges collected or induced in the pickup electrode 92 are impressed on the control grid 119. The alternating signal voltage thus provided is not applied to the second control grid 122 in view of the high resistor 132 placed in the series circuit interconnecting the two grids. The plate current of tube 116 flowing through the output transformer 146 as a result of this signal voltage will induce a voltage in each of the secondary windings 148 and 142. The voltage induced in the winding 148 is applied to the grid 126 of the triode section of the tube 117. When the output signal in transformer winding 148 exceeds a predetermined value, the positive signal peaks drive grid 126 positive and lowers the plate resistance of tube 117. This condition causes battery 133 to apply a more negative bias to grid 119 than battery 135. The diode section of tube 117 functions as a rectifier and prevents low voltage battery 135 from short circuiting high voltage battery 133.

There is thus applied across the leads 150 a voltage of constant amplitude resulting from the collected or induced charges on the pickup electrode 92. The phase of the signal voltage will be shifted with respect to the phase of the voltage applied across the voltage divider 99, 100, the degree of phase shift depending upon the spacing of the two electrodes and the velocity of the fluid medium carrying the ions. The distance between the two electrodes being known, a comparison of the two phases will give the velocity of the fluid medium.

To measure the phase shift of the signal voltages there is now provided the phase splitter circuit 151 and a dual channel amplifier generally designated by the numeral 155. The phase splitter circuit comprises a resistor 156 and a condenser 157 connected in series across the leads 153, the leads 159 being connected to the respective grids 158 and 159 of the two triodes 160 and 161 of the amplifier. The indirectly heated cathodes 162 and 163 of the amplifier tubes are connected through cathode resistors 164 to a grounded lead 165 connected to the junction point of the resistor 156 and condenser 15''] of the phase splitter circuit. A battery 166 forms the B+ supply for the plates 167 and 163 of the two triodes. The output circuit of the two triodes is impressed across the primary windings of two output transformers 170 and 171, the secondary windings 176a, 171a of which are connected to the grounded lead 165.

Due to the action of the phase splitter circuit 151, the signals impressed on the input of tubes 160, 161 are 90 out of phase. The amplified signals induced in the secondary windings 170a, 1719 of the transformers 170, 171 are also 90 out of phase with respect to each other.

The two out of phase signals are now impressed across the stator windings 172 and 173, respectively, of a two phase rotary induction device 174 of the inductive type resolver. The associated ends of the two windings are .i

connected by a lead 175 to grounded lead 165. A rotor winding 176, center-tapped to ground and adapted to be rotated by a two phase induction motor 177, will have a constant voltage induced therein whose phase will be dependent upon the angular disposition of the rotor winding with respect to the two stator windings.

The signal induced in rotor winding 176 is impressed across the diagonals of a phase discriminating bridge circuit 179 comprising four diodes 18th, 181, 182 and 183 connected in the four arms of the bridge to compare the phases of the signal voltage and reference voltage, and provide a D. C. voltage when the signals are other than 90 out of phase. The opposite diagonals of the bridge are connected by the leads 185 through suitable resistances 186 to the high side of resistor 100 of the voltage divider. These points on the bridge are further connected by leads 187 and 188 through filters including condensers 210, 211 and resistors 189, 190 to the input control grids 203, 204 of two parallel connected triodes 191 and 192.

The plates 195 and 196 of the two triodes are connected through the primary windings 197 and 198 of a saturable reactor 200 to B+ supply by the respective leads 201 and 202. The indirectly heated cathodes 206 and 207 are connected together through a cathode biasing resistor 208. A variable tap 209 for balancing the tube outputs is connected to ground and to the grid leads 187 and 188 through the condensers 210 and 211. The resistor 189 and condenser 210, and the similar elements 190, 211, form a filter circuit for removing any ripple appearing in the output circuit of the bridge 179, as already noted.

The saturable reactor 200 is provided with two secondary windings 204a and 205a connected in series and center-tapped to a lead 206a connected to a suitable source of A. C. potential 207a. The other ends of the windings 204a and 205a are connected by the respective leads 208a and 209a to the two phases 210a and 211a of the induction motor 177, the junction point of the two phases being connected by a lead 212 to the other terminal of the power supply 207a. A phasing condenser 213 is connected across the leads 203a, 209a to shift the phase of the current applied to the motor.

Through a suitable reduction gearing, herein indicated by the dashed line 214, the rotor 215 of the induction motor is coupled to the rotor 176 of the induction device 174. The rotor 176 in turn drives a pointer 217 over a calibrated scale 218 to indicate directly the velocity of the fluid medium measured, or, as in the case of a true airspeed indicator, the true airspeed.

Now the signal voltage induced in the rotor 176 of the rotary inductive device 174 as a result of the charge induced or collected by the pickup electrode 92 is applied across the diagonals of the bridge circuit 179. This voltage is compared by the bridge circuit as to phase with the phase of the voltage applied across the voltage divider 99, 190 by the amplifier 194. The reference voltage is applied to the opposite diagonals of the bridge circuit 179.

The signal voltage applied to the bridge circuit acts as a switching voltage which reduces the impedance of one half of the circuit, permitting the tubes 180 and 181 to conduct during one half cycle, and permitting the tubes 182 and 183 to conduct during the other half cycle of the signal voltage. The reference voltage applied to the bridge circuit from the voltage divider will thus flow through the conducting half of the bridge circuit. Since this voltage is applied at a 90 phase angle with respect to the signal voltage, when the phase of the signal voltage and that of the reference voltage is 90 out of phase, the bridge circuit will be balanced and no voltage will be impressed on either of the grids 203 or 204.

When the bridge circuit 179 is unbalanced by the reference voltages being other than 90 out of phase, a D. C. voltage will appear in the output circuit thereof which is responsive to the phase difference of the two applied voltages. This voltage is then applied to the grids of the triodes 191 and 192, the filter circuits 189, 210 and 190, 211 removing the unwanted ripples.

The plate currents flowing in the windings 197 and 193 of the saturable reactor are equalized by adjusting the cathode resistor 203. The value of the plate current is such that when the bridge circuit 179 is balanced the cores of the reactors are saturated. The circuit comprising the windings 204a, 205a and the phases 210a, 211a of the motor is balanced and the rotor 215 is at a standstill.

Upon unbalance of the bridge circuit 179, positive and negative voltages will be applied to the grids 203 or 204 of the balanced triodes, depending upon the degree of phase unbalance of the bridge circuit. The positive voltage on one grid and the negative voltage on the other will increase the impedance of one of the windings of the saturable reactor and decrease the impedance of the other winding. The motor 177 will thus be operated in a direction dependent upon unbalance of the bridge circuit 179. The rotation of the rotor 215 will position the rotor 176 of the inductive device 174 until the bridge circuit 179 is again balanced, i. e., when the reference volage and signal voltages are 90 out of phase. With the bridge circuit 179 again balanced, the plate currents in windings 197 and 198 will again be balanced, halting the operation of the motor 177.

The positioning of the rotor 176 will also position the pointer 217 with respect to the dial 218. Since the phase angle between the signal voltage induced in the pickup electrode 92 and the reference voltage is directly propor tional to the elapsed time for the ions to travel with the fluid medium from one electrode to another, the dial 218 may be laid off in units of linear distance per unit of time so that a reading of the pointer 217 will give the velocity of the fluid medium measured.

In the description hereinbefore given, the ionizing electrode 91 and the pickup electrode 92 were placed in the stream of the fluid medium of which the velocity is to be determined. lt has been found that the ionizing electrode may be placed adjacent to the fluid stream to provide the ionization required.

In the case of an airspeed indicator, the ionizing electrode would be recessed in a boom or within the body of the aircraft. The shaping of the electrode 91 and its insulating sheath and shroud are such that a narrow beam of ions is produced in the slipstream normal to the boom or aircraft skin. The pickup electrode 92 may be disposed in a similar recess, either open or covered over, the charge on the electrode being obtained by induction, rather than collection and induction as in the case where the probe extended into the airstream.

It will also be appreciated that the induced signal voltages impressed across leads 156 may be applied directly across the diagonals of the bridge circuit 179. In this instance, the voltage appearing across the resistor 100 would be applied to a phase splitter circuit 15L and applied to the two phases of the rotary inductive device 174.

The reference voltages induced in the rotor 176 would 1 then be applied to bridge circuit 179 to operate the pointer 217 in the manner described. The advantage of this method over that described is in the greater power available at the power amplifier 104 than across the secondary winding 14-2 of the compression circuit.

It will thus be seen that there is provided a velocity indicator in which the several objects of this invention are achieved, and which is well adapted to meet the conditions of practical use.

As various possible embodiments of the above invention might be made, and as various changes might be made in the embodiments above set forth, it will be understood that all matter herein set forth, or shown in the accompanying drawings, is to be interpreted as illustrative and not in a limiting sense. For example, other embodiments are illustrated and described in said Patent No. 2,679,162.

Having thus described my invention, I claim as new and desire to secure by Letters Patent:

1. A meter for measuring the velocity of a flowing fluid medium comprising an ionizing electrode, a source of periodically varying voltage connected to said electrode of suflicient potential to provide ionization of the fluid medium, a pick-up electrode at a known distance downstream from said ionizing electrode adapted to have a voltage induced therein by the ions carried downstream by the fluid medium, and a phase discriminator connected to said electrodes and responsive to the phase difference between the ionizing and induced voltages to indicate the velocity of the ions between the two electrodes.

2. A meter for measuring the velocity of a flowing fluid medium comprising an ionizing electrode, a source of periodically varying voltage connected to said electrode of sufficient potential to provide ionization of the fluid medium, means responsive to the pressure of the fluid medium for varying the voltage of the source for maintaining said ionizing electrode at pre-corona level, a pickup electrode at a known distance downstream from said ionizing electrode adapted to have a voltage induced therein by the ions carried downstream by the fluid medium, and a phase discriminator connected to said electrodes and responsive to the phase difference between the ionizing and induced voltages to indicate the velocity of the ions between the two electrodes.

3. A meter for measuring the velocity of a flowing fluid medium comprising an ionizing electrode, a source of periodically varying voltage connected to said elcc trode of suflicient potential to provide ionization of the fluid medium at pre-corona level, means responsive to fluid pressure for maintaining the ionizing potential at pre-corona levels as the ambient pressure of the fluid medium varies, a pick-up electrode at a known distance downstream from said ionizing electrode adapted to have a voltage induced therein by the ions carried downstream by the fluid medium, and a phase discriminator connected to said electrodes and responsive to the phase diflerence between the ioning and induced voltages to indicate the velocity of the ions between the two electrodes.

4. A meter for measuring the velocity of a flowing fluid medium comprising an ionizing electrode, a source of periodically varying voltage of suflicient potential to ionize the fluid medium at pre-corona level, a pick-up electrode at a known distance downstream from said ionizing electrode adapted to have a voltage induced therein by the ions carried downstream by the fluid medium, a balanced bridge circuit connected to said source and said pick-up electrode adapted to be unbal anced by a difference in the phases of the two voltages, and means operative by the unbalance of said bridge circuit to rebalance said circuit, the operation of said last means being responsive to the velocity of the fluid.

5. A meter for measuring the velocity of a fluid medium comprising an ionizing electrode, a source of periodically varying voltage of sufiicient potential to ionize the fluid medium, a pick-up electrode at a known distance downstream from the ionizing electrode adapted to have a voltage induced therein by the ions carried downstream by the fluid medium, a voltage limiter circuit connected to said pick-up electrode, a balanced bridge circuit connected to said source and the output of said limiter circuit, said bridge circuit being adapted to unbalance by a difference in the phases of the last two voltages, means responsive to the unbalance of said bridge circuit to r'ebalance the same, and means movable with said last means to indicate the velocity of the fluid.

6. A meter for measuring the velocity of a fluid iedium comprising an ionizing electrode, a source of periodically varying voltage of suflicient potential to ionize the fluid medium at pre-corona level, means rcsponsive to the ambient pressure of the fluid medium for maintaining the ionizing potential at pre'corona levels. apick-up electrode at a known distance downstream from the ionizing electrode adapted to have a voltage induced therein by the ions carried downstream by the fluid medium, a voltage limiter circuit connected to said pick-up electrode, a balanced bridge circuit connected to said source and the output of said limiter circuit, said bridge circuit being adapted to unbalance by a difference in the phases of the last two voltages, means responsive to the unbalance of said bridge circuit to rebalance the same, and means movable with said last means to indicate the velocity of the fluid.

No references cited. 

